The present article has the aim of showing how the particularity of the geographic enviroment of Chiloé was a conditioner of the Missionary Methodology that the Jesuits applied there between the XVII and XVIII centuries. The archipelago is composed by one main Island and more than thirty minor ones populated mainly by aborigines of the Huilliche tribe who were mostly bound by the Encomienda labour system. Spanish presence was concentrated in the town of Castro that lies on the eastern shore of the main Island and was founded on 1567. At that at the date was the southernmost urban enclave of the world. The Jesuits, using an uncommon system for permanent Missions, used the fying or Circular system of Mission as the core of what was considered a successful apostolate.